1. Field of the Invention
This invention relates to a method of manufacturing a molding having a core and a surface layer and showing a multilayered cross section by means of a double nozzle structure comprising an inner nozzle and an outer nozzle.
2. Background Art
A method of manufacturing a molding having a core and a surface layer and showing a multilayered cross section by means of a double nozzle structure comprising an inner nozzle for injecting a resin material to form the core of the molding and an outer nozzle for simultaneously injecting another resin material to form the surface layer of the molding is already well known. With such a double nozzle structure for manufacturing a multilayered molding, if the operation of resin injection is suspended simultaneously at the two nozzles, the resin material for forming the core partly remains in and out of the aperture of the inner nozzle. When the operation is resumed, the remaining resin becomes mixed with the resin material running through the outer nozzle for forming the surface layer so that, as schematically illustrated in FIG. 4 of the accompanying drawings, the resultant surface layer is made of a mixture of the resin material 2 for the surface layer and the resin material 3 for the core in areas near the gate of the injection molding system thereby failing to produce a multilayer molding.
Japanese Patent Application Laid-Open Publication No. Show 49-16539 discloses a method of manufacturing a double colored molding that can improve such a damaging phenomenon by reducing the pressure being applied to the inner nozzle when the injecting operation is suspended in order to cause the resin remaining outside the inner nozzle to be driven back into it by the pressure being applied to the outer nozzle.
[Problems to be Solved by the Invention]
However, with the above improved method of manufacturing a multilayer molding, the resin material for forming the core can readily flow out of the inner nozzle while it is being volumetrically measured for another injecting operation so that the resin to be injected from the inner nozzle cannot be measured when resin is being injected from the outer nozzle under pressure in order to maintain the inner pressure of the double nozzle structure. Additionally, if a foaming resin material is used for the core, measures need to be taken to prevent adverse effects of the pressure of the resin foam injected into the mold cavity on the resin remaining in the nozzles.
While these problems are solved by providing the inner nozzle with a shut-off pin, since the shut-off pin is so designed as to close the aperture of the nozzle the instant that the injecting operation is suspended to shut off the aperture of the nozzle and the inside of the nozzle, it cannot be feasibly used with the above described improved method of causing the resin remaining outside the inner nozzle to be driven back into it by utilizing the pressure difference between the inner and outer nozzles.
With a proposed technique, the operation of resin injection for each molding is terminated at a timing earlier than that of the common practice to make the amount of resin driven out of the outer nozzle for each molding somewhat excessive relative to the corresponding amount injected from the inner nozzle so that the excessive resin can be flushed to clear the inner nozzle. However, with this proposed technique, the core of the produced molding can contain to certain extent the resin material for the surface layer, which may produce shrinkage and other defects on the product and the product can put on an excessive weight. Additionally, if waste plastic is used for the core, this does not provide a recommendable way to effectively consume waste plastic because the mold is filled with less waste plastic.
In view of the above circumstances, it is therefore an object of the present invention to provide a method of manufacturing a multilayer molding that eliminates any adverse effects of the resin material for forming the core of the molding remaining in the inner nozzle in order to make the molding flawless even in areas near the gate of the molding system if the inner nozzle of the molding system is provided with a shut-off pin.